1. Field of the Invention
The present invention relates to an air-fuel ratio controlling device for an internal combustion engine, and particularly to an air-fuel ratio controlling device for an internal combustion engine having an exhaust system which is provided with a NOx (nitrogen oxide) absorbent catalyst.
2. Description of the Related Art
When the lean operation is executed in such a way that the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine is set to be on the lean side with respect to a theoretical air-fuel ratio, the output amount of NOx tends to be increased.
Conventionally, a technique is therefore known in which a NOx absorbent catalyst that absorbs NOx is disposed in an exhaust gas passage of an engine so as to purify an exhaust gas. Such a NOx absorbent catalyst has properties such that, in a state where the air-fuel ratio is set to be on the lean side with respect to a theoretical air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high (NOx is rich) (hereinafter, referred to as "exhaust gas-lean state"), the catalyst absorbs NOx, and, on the other hand, in a state where the air-fuel ratio is set to be on the rich side with respect to the theoretical air-fuel ratio, the oxygen concentration in the exhaust gas is low, and the HC and CO components are large (hereinafter, referred to as "exhaust gas-rich state"), the catalyst releases NOx which has been absorbed.
Conventionally, an air-fuel ratio controlling device is known in which such a NOx absorbent catalyst is disposed together with a three-way catalyst in an exhaust gas passage of an engine and the air-fuel ratio of an air-fuel mixture is periodically controlled to be on the leaner side and the richer side with respect to a theoretical air-fuel ratio (Japanese Patent No. 2,605,579). In the device, in view of a phenomenon that, when the air-fuel ratio is set to be on the leaner side with respect to the theoretical air-fuel ratio, the NOx purification rate of the three-way catalyst is lowered, the NOx absorbent catalyst is placed downstream from the three-way catalyst so as to absorb NOx.
In a device of the related arts, however, no consideration is given on reduction (release) of NOx which has been absorbed by the NOx absorbent catalyst, and hence there is the following problem.
When the air-fuel ratio is to be controlled so as to be in the vicinity of the theoretical air-fuel ratio, an air-fuel ratio on the richer side with respect to the theoretical air-fuel ratio is set a value at which HC and CO can be purified by means of the purification property of the three-way catalyst. On the downstream side of the three-way catalyst, therefore, the exhaust gas-rich state is not attained usually. Consequently, the reduction of the absorbed NOx in the NOx absorbent catalyst is insufficiently performed, thereby there is such a problem that the NOx absorption ability is gradually lowered and the output amount of NOx is increased.